Conversion of oil-base mud to oil mud-cement

ABSTRACT

A water-in-oil emulsion drilling fluid can be converted into an oil mud-cement slurry for use in oil well cementing procedures. Also, a universal fluid can be prepared using the water-in-oil emulsion drilling fluid by treating with an hydraulic material so that it will lay down a filter cake during drilling which can be triggered to set into a hard cement and bond to the formation after the above oil mud-cement slurry has been placed in the borehole at the conclusion of drilling.

This is a continuation of application Ser. No. 08/033,404, filed Mar.19, 1993, which is a division of application Ser. No. 961,217 filed Oct.15, 1992, which is a continuation of application Ser. No. 691,904 filedApr. 26, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to improvements in compositions andmethods for zonal isolation for vertical, deviated, and horizontal oiland gas wells. The method involves the utilization of oil-base universalfluid for providing settable filter cake while drilling and thesubsequent in-situ solidification of the drilling mud or both thedrilling mud and universal fluid filter cake to compressive strengthswell in excess of that required for casing support, zonal isolation, andborehole stability.

2. Description of Prior Art

The general procedure of drilling an oil or gas well includes drilling aborehole using a drilling fluid (also termed drilling mud). Subsequentto drilling the borehole, casing is run into the well and a cementslurry is then placed in the annulus between the outside of the casingand the borehole wall. In order to obtain a good cementing job, it isnecessary for the cement slurry to displace substantially all of thedrilling mud from the annulus. A reduced displacement efficiency arisesfrom the geometry of the system and also from the fact that drillingfluids and cements are usually incompatible.

Nondisplaced mud (mud still left in the borehole after cementing) andmud filter cake are major causes of unsatisfactory cement performance.Since the nondisplaced mud and mud filter cake do not set or bond to thecasing, the borehole wall or the set cement itself, the mud and filtercake do not support the casing properly and later can allow annular gasor fluid migration.

Wells frequently are drilled with oil-base muds which contain water asthe internal emulsified phase. These oil-base muds are used, forexample, to lower drilling torque and drag, reduce damage to productiveformations, increase wellbore stability or reduce differential pressurepipe sticking, etc. The presence of oil mud in the wellbore can evenfurther reduce the displacement efficiency while cementing with aconventional water-base cement slurry.

The drilling industry has sought to overcome the above problems by usinga variety of techniques to displace the drilling fluid with cement,e.g., turbulent flow, casing movement (reciprocation/rotation), casingequipment (centralizers, flow diverters, mud cake scratchers), andspecial spacers and wash fluids, but these have had limited success.Even greater cementing difficulties are encountered when the boreholesdeviate greatly from vertical. Major problems arise in connection withcasing placement, drilled solids settling, mud displacement, casingcentralization, fluid separation (free water), and mechanical friction.When a good cementing job is not obtained, it may be necessary toperforate the casing and squeeze cement under high pressure through theperforations into the annulus and try to fill the zones that were notproperly cemented initially. Frequently, this squeeze cementing is notsuccessful, and such failures may eventually lead to abandoning thehole.

One of the major objectives of a primary cementing is to obtain goodzonal isolation in the annulus of the well. Effective zonal isolation isachieved by sealing the cement and borehole wall. The interface of thecement and borehole wall is usually an interface between the cement anddrilling fluid filter cake which is the source of many cementingproblems. Good zonal isolation can be achieved if the filter cakehardens, permanently bonds to the formation face and the cement, andprovides hydraulic sealing.

Accordingly, the present invention is directed to overcoming theabove-noted problems in the art and to providing a solution as moreparticularly described hereinafter.

SUMMARY OF THE INVENTION

The primary purpose of the present invention is to provide compositionsand methods for in-situ solidification of oil-base drilling muds and forforming oil-base universal fluid filter cakes which can be solidified.

Hereinafter, the term "cement" is used in reference to Portland cementor other available hydraulic materials. Most of the exemplificationherein is based on using Portland cements. The type of oil in theoil-base mud is usually not critical to the cementing process in thepresent context. If necessary, a satisfactory oil mud cement slurrycould be made from pure oil by adding sufficient amounts of cement,water, emulsifiers, fluid loss additives, suspending agents,accelerators, etc.

Drilling muds are used for the present in-situ conversion process formany reasons. The oil-base mud is already present at the well site inthe large quantities necessary for cementing the well. All (or most) ofthe emulsifying, stabilizing, and fluid loss control materials, and amajor portion of the mechanical shearing necessary to produce a goodemulsion already have been incorporated into the mud to be converted toan oil mud cement slurry. It is much easier, less expensive, and lesstime consuming to make adjustments to a preexisting invert emulsion mud(oil-base mud) than to build one from scratch.

In addition, drilling mud left after the drilling of a well must bedisposed. Oil-base muds are even more of an environmental problem thanare water-base muds. If a large portion of the leftover drilling mud isused to cement the well, the total disposal expense can be substantiallyreduced.

A satisfactory oil mud cement slurry must have a sufficiently high ratioof total solids-to-oil so that the solids particles will be closelyspaced, and growing cement hydrate crystals can link to each other andadjacent solids particles to give a strong, unified, cohesive structurewithout the separation of oil from the solid mass. The total solids(cement plus the solids already in the mud)-to-oil volume ratio can varyfrom about 1.4 to 2.2.

Sufficient water must be present in the slurry to allow for hydration ofthe dry cement particles. Extra water is normally added even to theslurries prepared from high water content oil-base muds. This is toprovide hydration water for the total cement added to satisfy thedesired solids-to-oil ratio. Thus, the present slurry design is atwo-step process: first, enough cement, and second, enough water.

Drilling muds having a low oil-to-water ratio (high water) are preferred(but not necessary) according to the following reasoning. High watermeans lower oil content, so less cement is required to provide thecorrect solids-to-oil ratio. This lowered cement content then requiresless additional water for hydration. If a stable high-water oil-base mudis used, additional emulsifiers probably will not be required. Low-wateroil-base muds usually require additional emulsifiers and stabilizers toproduce a good oil mud cement. Thus lower added volumes and costs arerequired for each barrel of the excess high-water drilling mud used.Also more barrels of the drilling mud are used to produce the requiredfluid volume of oil mud cement and the ultimate disposal costs arereduced.

In conjunction with the method and composition for the solidification ofan oil-base drilling mud, the present invention includes a compositionand method for drilling with an oil base universal fluid and forproviding a filter cake (formed as a permeable formation is drilled withan oil-base mud) which can be solidified to form a cementitiousstructure intimately bound to the formation itself.

Accordingly, the present invention provides a method for cementing awell comprising admixing an oil mud with (a) sufficient hydraulicmaterial and water to form a slurry having a total solids-to-oil volumeratio of about 1.4 to 2.2 and a water-to-hydraulic material weight ratioof about 0.15 to 0.60, and (b) sufficient accelerator or retarder tocause the oil mud cement to harden and set up at a preselected time; anddisplacing the slurry to a preselected location in the well, andallowing the oil mud cement to harden and set up.

The present invention also provides a composition for cementing a wellcomprising an oil mud containing sufficient hydraulic material and waterto form a slurry having (a) a total solids-to-oil volume ratio of about1.4 to 2.2 and a water-to-hydraulic material weight ratio of about 0.15to 0.60, and (b) sufficient accelerator or retarder to cause the oilmud-cement to harden and set up at a preselected time.

Other purposes, distinctions over the art, advantages and features ofthe invention will be apparent to one skilled in tile art upon review ofthe following.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following preferred embodiments of the invention, together with thefollowing examples, explain the principles of the invention.

In accordance with the present invention, it has been discovered thathigh water oil-base muds, with an alkali metal halide (preferably sodiumchloride) internal phase, are particularly suitable for forming oil mudcements. Although calcium chloride brine muds usually are preferred infield drilling operations, it has been found that it is generally notsatisfactory to use calcium chloride in the formation of an oil mudcement. Also, although not preferred, conventional invert oil muds,e.g., having an oil-to-water volume ratio of 95:5 to 70:30, also can beused for in-situ mud conversion, although additional emulsifiers andstabilizers may be required. By the term "high water" it is intended tomean an oil-to-water volume ratio of approximately 30:70 to 70:30, witha ratio of about 50:50 being optimal. The high water oil-base muds mustbe formulated to have high emulsion stability with large amounts ofwater under high temperature drilling conditions. Such muds musttolerate the high water content and maintain reasonable rheologicalproperties comparable to those of regular invert oil muds. A highlycontamination-resistant mud is desired because a large quantity ofcement and extra water are added to the mud system in order to convertthe mud into a high-compression strength mud concrete. The addition ofany necessary emulsifiers, stabilizers or wetting agents adds to thecost of the system.

One barrel of a representative 13.5 lb/gal mud suitable for use in theinvention has a composition as follows: diesel oil, 0.42 barrels; lime,2.4 pounds; "CARBO-TEC L" (primary emulsifier, tall oil fatty acids, byMilpark Drilling Fluids), 1 gallon (8.1 pounds) water, 0.42 barrels;sodium chloride, 49 pounds; "CARBO-MUL" (secondary emulsifier, tall oilpolyamide, by Milpark), 1 gallon (7.7 pounds) "SURFCOTE" (wetting agent,amine dodecyl benzene sulfonate, by Milpark), 2.0 pounds; barite, 312pounds. While the preceding mud formulation is with diesel oil, it is tobe understood that other oils or hydrocarbons may be employed. The useof diesel oil-based muds is restricted in many parts of the worldbecause of environmental concerns. Non-toxic, mineral or "clean" oilmuds offer the benefits of an oil-base mud while maintaining a toxicitynear that of water-base muds. Currently, non-toxic mineral oils such asExxon's "ESCAID 110" (1.6 csf @ 40° C.), Conoco's "LVT 200" , Vista's"ODC", and Shell's "SOL DMA" are available. These oils are considerablythinner than diesel oil (3.7 csf @ 40° C.), but can be usedsatisfactorily for making low toxicity oil-base muds.

As above mentioned, it is highly preferred to use a sodium chloride saltwith the oil mud of the present invention. Calcium chloride brine mudsare very difficult to use, giving many thickening, emulsion-breaking,and fluid separation problems even while mixing. The calcium chlorideslurries become very thick during mixing and never attain a satisfactoryset strength for use herein. Thus, although calcium chloride brine mudsusually are preferred in field drilling operations, they are notdesirable for making the oil mud cements. Other less preferred saltswhich may be suitable for use with the invention include potassiumchloride, zinc chloride, sodium nitrate, and ammonium sulfate.

As also above mentioned, the present invention utilizes an oil base mudto which is added cement and extra water if desired for additionalcement hydration. The dry cement particles are coated with the oil phaseand the extra water is dispersed as the internal emulsified phase(adding to the emulsified water already present in the mud).

The water becomes available to the cement for hydration by anosmosis-like process across a semi-permeable membrane of oil andsurfactants between the water droplets and the cement particles. Anosmotic pressure causes water molecules to migrate across thesemi-permeable membrane from the high vapor pressure (wet) waterdroplets to the low vapor pressure (dry) cement particles. Watermolecules arriving at the cement combine with the "water thirsty" cementparticles to form calcium silicate hydrate crystals. The calciumsilicate hydrate crystals grow and extend through the surrounding oilphase, link together, and form a competent set cement structure. As longas the vapor pressure of the cement particles remains low, the osmosisof water occurring in the cement hydration reactions will continue. Themajority of water present in the mud is consumed by the cement hydrationreactions, and the emulsion droplets are used up as the oil mud cementsets. The water-internal emulsion does not break during setting. The oilremains evenly dispersed in the set mass, and no water or oil separatesfrom the set oil mud cement. Any separated fluids would produceundesirable channels in the set oil mud cement.

The hydration reactions are accelerated by increased temperature, whichincreases the chemical potential (vapor pressure driving force), and byhigh mechanical shear during preparation and well operations. Thismechanical energy helps to generate a large number of small waterdroplets which lie closer to the cement particles. The cementaccelerators increase the rate of early strength development bydecreasing the dormant or induction period in the hydration of portlandcement.

While various waters may be used with the present invention, such asfresh water, brackish water, brine, seawater or other water-containingfluids, for ease of control it is preferred that fresh water added toallow full hydration of the cement be used. Waters other than freshmight have various materials (e.g., salts, lignins or tannins) presentwhich could affect the cement hydration rates.

Not only is the type of water employed significant, but it is alsoconsidered important that the correct amount of water be utilized. Alarge amount of cement is added to the drilling mud to make a slurrythat can attain the desired set strength. Water is necessary for thecement hydration which produces the set strength. If enough water is notalready present in the stable oil-base mud, it must be added. Awater-to-cement weight ratio of about 0.15 to 0.60 is preferred andoptimally about 0.30 to 0.35. If a high-water oil-base mud is used, themud has already been conditioned to withstand a high water content andit will probably remain stable when the cement and extra water (ifnecessary) is added without requiring treatment with expensiveemulsifiers, stabilizers or wetting agents. Thus the oil-base mud withan oil-to-water volume ratio of 30:70 to 70:30 has been specified with apractical optimum of about 50:50. Water in excess of the amount neededto hydrate the cement will increase the viscosity (as water is theinternal phase), reduce the slurry density, and remain unreacted in theset oil mud-cement mass, thus reducing compressive strength andincreasing porosity and permeability. Theoretically, the water requiredfor complete cement hydration is about 25% of the weight of cement. Innormal practice oil well cementing, water-to-cement values of 35% to 45%by weight are used to provide sufficient fluidity and density control.Hereinafter, a water-to-cement value of 35% by weight usually has beenused to be sure that complete cement hydration takes place.

Another significant variable of the present invention is the amount ofsolids in tile oil mud-cement slurry. A solids-to-oil ratio by volume of1.4 to 2.2, and optimally 1.80 is preferred. Any oil in excess of theamount needed to coat all of the added cement particles and to providesufficient fluidity for mixing and pumping will remain as an inertdiluent in the set oil mud cement mass, reducing the compressivestrength and giving increased porosity and permeability. By using aconstant volume ratio of the total solids (mud solids pluscement)-to-oil, a nearly comparable fluidity is attainable using any oilmud. The amount of oil is fixed in a given base mud and no extra oil isrequired. A solids-to-oil volume ratio of 1.8 appears to be optimum.From this solids-to-oil volume ratio value, the amount of cement neededis determined. Then, based on a chosen water-to-cement ratio, the amountof supplemental water is calculated.

Another important variable in the present invention is the cementmaterial which is employed. In accordance with the present invention ithas been found that Portland cement is economical, is widely availableas a finely ground powder, and usually is very suitable for use. Otherhydraulic materials such as fly ash, natural pozzolan, trass, tuff, andblast furnace slag may be usable for these purposes. None of theselatter agents, however, performs as well as Portland cement which ispreferred. Among the Portland cements, Class C or A cement gives higherstrength than does Class G or H, and accordingly, Class C or A isgenerally preferred.

It has been found that oil mud-cements set harder and more rapidly at180° F. than at 150° F. Thus, to the extent that temperature iscontrollable, it is possible to control the setting rate of the oilmud-cement. Normally, the control of temperature in a well is not withinthe scope of the operator, at least to any significant degree.

Essentially, oil mud-cements preferably are prepared by adding aPortland cement to an oil-base mud. An emulsified internal water phaseis necessary to allow hydration of the cement. Supplemental water may beemulsified in the system to provide more hydration. The cement dispersesin the oil phase and remains out of contact with tile water under normalconditions of mixing and storage. With time and/or higher temperatures,the water becomes available in the cement, and the cement hydrationreaction proceeds, giving a solid set mass.

The slurry viscosity, the setting time, and the set strength of theslurries of the present invention can be adjusted using appropriateoil-wetting agents, emulsifiers, and accelerators, as well ascontrolling temperature. The mechanism by which the setting occurs isnot fully understood, but with increased temperature, the rate ofsetting is greatly increased. Initially, the mud should be able toaccept addition of the cement and extra water while maintaining stableproperties, so that the resulting slurry can be mixed and pumped into awellbore. With time at elevated temperatures, the water moleculesapparently migrate through the oil phase to the cement particles by anosmosis-like process. The cement then slowly hydrates, and the calciumsilica hydrate crystals grow through the oil phase to link and form asolid structure. The oil and any unreacted water remain evenly dispersedin the set cement. Too little water will not produce sufficienthydration to give strength, while too much water unnecessarily increasesthe porosity and permeability and reduces the strength. Any oil presentin excess of that required to coat all of the solid materials and toprovide sufficient fluidity remains in the solidified mass, increasingthe porosity and permeability and reducing the ultimate strength.Properly prepared oil mud-cements have very low air permeabilities.

Another significant variable of the invention is the additives employedin the system. These various additions, used in effective amounts, canprovide set acceleration, retardation, or viscosity control as well asadditional emulsification or oil wetting properties if needed for theincorporation of tile cement or extra water.

Set accelerators are desirable to provide sufficient set strength withina reasonable time. "POZZUTEC-20" (a proprietary commercial lowtemperature accelerator of mixed organic and inorganic salts made byMaster Builders for the construction industry) is an effectiveaccelerator. By itself, FAM (an acronym for a mixture of 45% by volumeformic acid, 45% by volume acetic acid, and 10% by volume methanol) isnot as effective as Pozzutec-20 but it seems to work very well incombination with the Pozzutec-20. Formic acid and acetic acid separatelygive some acceleration but appear to work more effectively incombination and with methanol. MSF (magnesium silicofluoride) and ZSF(zinc silicofluoride) appear to be about equivalent but not as effectiveas either FAM or Pozzutec-20. "SURFCOTE" (a Milpark oil wetting agentbelieved to contain dodecyl benzeneaminesulfonate) is the only agentfound to have a thinning effect on the oil mud cement other than thethinning effect afforded by mere oil dilution. However, SurfCote is alsoa strong set retarder.

To help stabilize the high water content of the oil mud cements,inclusion of additional emulsifiers may be required. Milpark's primaryemulsifier "CARBO-TEK L" (primarily a modified tall oil fatty acid),their secondary emulsifier "CARBO-MUL" (known as tall oil polyamide),and their "CARBO-DRILL" (composition undisclosed), an experimentalemulsifier for high water content oil muds, have been useful for thispurpose.

The above-mentioned emulsifying and stabilizing agents are used to givethe oil mud-cement stability for whatever conditions are necessary. Itis preferred, however, not to overtreat the oil mud-cement system sincesuch agents are expensive and frequently are strong set retarders. Anyover-retardation then requires additional accelerators to give a setwithin the desired time. For example, three similar oil-base muds(designated Na-1, Na-2, and Na-3 ) were mixed in the laboratory. Na-1had moderate amounts of the emulsifiers and stabilizers; Na-2, excessiveamounts Na-3, minimal amounts. Oil mud-cement slurries of each, made byadding the same calculated ratios of cement-to-solids andwater-to-cement, were then placed in a 180° F. oven to cure. Oilmud-cement prepared from Na-1 became hard in five days oil mud-cementprepared from Na-2 did not become hard even by 36 days and oilmud-cement prepared from Na-3 became hard by one day. The addition of7.5 lb/bbl of Pozzutec-20 made both oil mud-cements made from Na-1 andNa-2 also become hard in one day.

EXAMPLES Laboratory Test Procedures

Screening tests were conducted in small glass vials. A base oilmud-cement slurry was made by adding cement, extra water, and anyadditives to the oil muds using normal lab mixers and Hamilton-Beachhigh-speed mixers. Equal portions (20 ml) of this base slurry were thenmeasured into small glass vials (32 ml) for test series.

The samples were sealed in the vials and heat aged statically forhardening. The large majority of the samples were aged at either 150° F.or 180° F. The setting of the oil mud-cement (OMC) samples weremonitored with time using a spatula "hardness" test in the vials. Thehardness scale ranged from 0 to 10, with 0 being completely fluid and 10being "rock hard." A hardness of 9 corresponds to a compressive strengthof approximately 150 psi. Samples with hardness greater than 9 wereretested using a Brinell tester to estimate equivalent compressivestrengths measured in psi. All laboratory testing used Milpark's 13.5lb/gal CarboDrill-HW oil mud (oil-to-water ratio of 50:50) with a 25percent sodium chloride brine as the internal phase. The water added toallow full hydration of the cement was fresh water rather than brine.

Table A shows the effects of changing the solids-to-oil ratio studied inthe absence of accelerators using a base mud system, Na-2. The amount ofClass A cement needed to give the same solids-to-oil ratio varies withmud composition. Higher solids-to-oil ratios seem to produce slightlyhigher compressive strength.

TABLE A Solidification of Various 14 lb/gal CarboDrill-HW Muds WithoutAdding Any Accelerator: Effect of Maintaining a Constant Water-to-CementRatio and Varying Total Solids-to-Oil Ratio (SOR) on CompressiveStrength of Samples Cured Static at 180° F.

                                      TABLE A                                     __________________________________________________________________________    Solidification of Various 14 lb/gal CarboDrill-HW Muds Without                Adding Any Accelerator: Effect of Maintaining a Constant                      Water-to-Cement Ratio and Varying Total Solids-to-Oil Ratio (SOR)             on Compressive Strength of Samples Cured Static at 180° F.                          Class A    Hardness                                                                           Compressive                                             Mud   Cement                                                                             Water w/Time                                                                             Strength                                         Slurry Type                                                                             SOR                                                                              (lb/bbl)                                                                           (lb/bbl)                                                                            (in days)                                                                          (psi) (GHN)                                      __________________________________________________________________________    OMC 358                                                                              Na-2                                                                             1.85                                                                             617.5                                                                              129.5 3.5                                                                             (24),                                                                            ns                                                                       3.8                                                                             (36)                                                OMC 359                                                                              Na-2                                                                             2.0                                                                              688.0                                                                              157.5 3.7                                                                             (9),                                                                             ns                                                                       3.7                                                                             (28),                                                                       4.4                                                                             (36)                                                __________________________________________________________________________     Notes:                                                                        SOR, cement and mud solidsto-oil ratio.                                       40% bwoc (by weight of cement) water (initial and added) used in all          cases.                                                                        *ns = not set                                                            

The samples prepared using the Na-2 mud were strongly retarded and didnot set. The Na-2 mud contained both SurfCote, a powerful oil wettingagent, and CarboDrill, an experimental emulsifier of undisclosedchemical composition.

The two slurries (OMC 358 and 359) shown in Table A were used as baseslurries to study the effectiveness of several accelerators. The settingof base slurries OMC 358 and 359 using the highly retarded Na-2 mud wasaccomplished only by adding 7.5 lb/bbl Pozzutec 20.

A series of tests screening candidate accelerators was conducted at 150°F. and 180° F. The hardness test results are listed in Table B. APIClass A cement at 600 lb/bbl levels was added to specially formulatedhigh-water oil emulsion muds, Na-2, along with several accelerators andsetting agents. Accelerators are added to the OMC slurries to speed thereactions and to attain a higher set strength within a reasonable time.They were Pozzutec 20 (PT-20), magnesium silicofluoride (MSF), and zincsilicofluoride (ZSF). Pozzutec 20 is a low temperature accelerator fromMaster Builders used in the construction industry.

Table B shows that (1) a temperature increase from 150° F. to 180° F.increases the compressive strength, (2) a PT-20 (Pozzutec 20)concentration of 7.5 lb/bbl seems to be about optimum, and (3) MSF andZSF appear to be about equivalent, but Pozzutec 20, at a concentrationof 7.5 lb/gal appears to be superior to either of them.

                  TABLE B                                                         ______________________________________                                        Effect of Temperature and Accelerator on Compressive                          Strength of Set 14 lb/gal CarboDrill Na-2 Mud                                                    Hardness/    Hardness/                                                        Compressive  Compressive                                                      Strength     Strength                                              Accelerator                                                                              (BHN)        (BHN)                                         Slurry  (lb/bbl)   Aged at 150° F.                                                                     Aged at 180° F.                        ______________________________________                                        Base     --        3.51--       --                                            OMC 1   MSF, 6     9.5/--        10/570                                       OMC 2   MSF, 9      9.7/250      9.8/410                                      OMC 2a  ZSF, 9      9.9/440      10/280                                       OMC 3   ZSF, 9      9.7/410      9.9/850                                      OMC 4   PT-20, 5.5  9.9/480      10/840                                       OMC 5   PT-20, 7.5  10/520       10/890                                       OMC 6   PT-20, 10   10/560       9.8/710                                      OMC 7   PT-20, 15   10/590       9.9/710                                      ______________________________________                                         Notes:                                                                        Base oil mudcement contains 600 lb/bbl Class A cement and 94.5 lb/bbl         supplemental water                                                            MSF, magnesium silicofluoride                                                 PT-20 is Pozzutec 20 by Master Builders                                       ZSF is zinc silicofluoride                                               

Efforts were continued to improve the compressive strength by usingcombinations of additives. Table C shows that OMC 393 had a compressivestrength of 1,300 psi. This was achieved by adding 7.5 lb/bbl Pozzutec20 and 1.75 lb/bbl cyclohexanol in a Na-2 base mix which contains 600lb/bbl Class A cement and 107 lb/bbl additional water. In this case, thecyclohexane doubled the strength of a comparable slurry, but in twoother cases it slightly decreased the strength, and in the fourth caseit completely prevented set. Although the role of cyclohexanol was notclear, it may have acted as a mutual solvent such that the cementhydration reactions were improved.

                  TABLE C                                                         ______________________________________                                        Evaluation of Accelerators                                                    and Other Additives in Na-2 Mud: Hardness and                                 Compressive Strength of Samples Cured Static at 180° F., Base          OMC Contains 600 lb/bbl Class A Cement and 93 lb/bbl Water                                                     Compressive                                           Accelerator/Additive                                                                         Hardness Strength                                     Slurry   (lb/bbl)       (days)   (psi) (BHN)                                  ______________________________________                                        OMC 248  None            3.0 (64)                                                                              n.s.                                         OMC 383  PT-20, 7.5     9.9 (3)  690                                          OMC 384  PT-20, 3.8     9.8 (3)  750                                          OMC 385  PT-20, 7.5     9.9 (5)  760                                          OMC 386  W, 13.3; PT-20, 7.5                                                                          9.9 (5)  730                                          OMC 387  W, 26.6; PT-20, 7.5                                                                          9.9 (5)  550                                          OMC 391  PT-20, 7.5; CH, 0.9                                                                          9.9 (4)  750                                          OMC 392  PT-20, 7.5; CH, 1.8                                                                          9.9 (4)  640                                          OMC 393  W, 13.3; PT-20, 7.5                                                                           4.4 (4),                                                                              1300                                                  CH 1.75         9.8 (11)                                             ______________________________________                                         Notes:                                                                        CH, cyclohexanol                                                              W, water                                                                      n.s., did not set or no strength                                         

As shown in Table D, the effects varying amounts of extra water had onhardness and compressive strength were investigated in the Na-2 mud. Theextra water varied from 0 to 209 lb/bbl, while Pozzutec 20 concentrationwas kept constant. OMC 411 gave a compressive strength of 1,057 psi with93 lb/bbl of extra water. This strength peak was at a W/C ratio of 35percent, rather than at the 25 percent ratio, which theoretically cangive complete hydration.

                                      TABLE D                                     __________________________________________________________________________    Solidification of Na-2 Mud: Effect of External Water                          At 180° F.; Base Mix = Na-2 + 600 lb/bbl Class A Cement                                                BHN                                                 Added                                                                              Total                                                                             Additive/        Compressive                                         Water                                                                              Water                                                                             Concentration                                                                         Hardness/Curing                                                                        Strength                                      Slurry                                                                              (lb/bbl)*                                                                          (bwoc)                                                                            (lb/bbl)**                                                                            Time (days)                                                                            (psi)                                         __________________________________________________________________________    OMC 409                                                                              0   19.6%                                                                             PT-20, 13.9                                                                           6.0 (3), 8.5 (4),                                                                      852                                                                  9.7 (6)                                                OMC 410                                                                              70  31.3%                                                                             PT-20, 13.9                                                                           9.9 (3)  925                                           OMC 411                                                                              93  35.1%                                                                             PT-20, 13.9                                                                            10 (3)  1057                                          OMC 412                                                                             140  43.0%                                                                             PT-20, 13.9                                                                           9.9 (3)  911                                           OMC 413                                                                             209  54.4%                                                                             PT-20, 13.9                                                                           9.9 (3)  727                                           __________________________________________________________________________     *Concentration based on mud.                                                  **Concentration based on base mix (mud + cement + water).                     BHN is Brinell hardness number                                           

Table E lists the test results of Na-4 and Na-5 (conventional low wateroil mud) muds. The Na-5 mud is a 14.3 lb/gal Milpark CarboDrill with anoil-to-water ratio of 87:13. The Na-5 mud is not a "high water" mud.First of all, the base mix 449 did not set. OMC 451 made from the Na-5mud set hard and both contained 7.5 lb/bbl Pozzutec 20. The effect ofFAM on set acceleration is not as good as that of Pozzutec 20.

                                      TABLE E                                     __________________________________________________________________________    Solidification of Na-5 Muds at 180° F.:                                Na-5 Mud-Milpark's 14.3 lb/gal CarboDrill (Oil-to-Water Ratio 87:13)                                        BHN                                                      Additive/            Compressive                                           Base                                                                             Concentration                                                                          Hardness/Curing Time                                                                      Strength                                        Slurry                                                                              Mix                                                                              (lb/bbl) (days)      (psi)                                           __________________________________________________________________________    OMC 449                                                                             449                                                                               --      3.7 (8)     --                                              OMC 451                                                                             449                                                                              PT-20, 7.5                                                                             5.2 (1), 9.2 (4)                                                                          407                                             OMC 453                                                                             449                                                                              FAM, 4   3.7 (1), 9.6 (4d), 9.7 (8)                                                                --                                              OMC 455                                                                             449                                                                              FAM, 6   5.8 (1), 9.6 (4), 9.7 (8)                                                                 278                                             OMC 457                                                                             449                                                                              SC*, 1.9; PT, 7.5                                                                      7.0 (4) , 9.8 (8)                                                                         <237                                            OMC 459                                                                             449                                                                              SC, 1.9; FAM, 4                                                                        9.7 (4d)    <237                                            __________________________________________________________________________     449 = Na5 + 921 lb/bbl Class A cement + 335 lb/bbl water; solidsto-oil        ratio = 1.8; waterto-cement = 40%                                             *SC = SurfCote                                                           

The base slurry OMC 449 contains 921 lb/bbl Class A cement and 335lb/bbl water in order to maintain a solids-to-oil ratio of 1.8 and awater-to-cement ratio of 40 percent. This will result in a volumeincrease of 180 percent based on the original mud volume. It is quiteclear that a considerable amount of cement is needed to absorb the largeamount of oil present in the Na-5 (conventional) oil mud. Considerableamounts of extra water are also needed to provide sufficient water forthe cement hydration.

The retarding effect of SurfCote was demonstrated in a series ofexperiments shown in Table F. Addition of SurfCote clearly reduces theslurry yield point (lb/100 ft²), but it retards the setting. Theretardation by SurfCote can be reversed by adding an accelerator such asPozzutec 20. The OMCs treated with SurfCote did not set without addingPozzutec 20. Table F clearly shows that the more SurfCote added, themore Pozzutec 20 is needed to offset the influence of its retardation.

                  TABLE F                                                         ______________________________________                                        Effects of Pozzutec 20 on Setting and Compressive Strength                    of an OMC Containing Varying Amounts of SurfCote                                                  Compressive Strength, psi                                 SurfCote Rheology   Pozzutec 20, lb/bbl                                       (lb/bbl) YP         0      7.5    13.1 18.75                                  ______________________________________                                        2.0      330         ns*   1107   1115  930                                   3.0      285        ns     ns      913  977                                   5.0      250        ns     ns     ns   1008                                   3.0/2.0**                                                                              250        ns     ns     ns   1143                                   ______________________________________                                         *ns = not set                                                                 **Added separately two different times                                   

High Water Oil-Base Mud System

A 12-barrel batch of a 13.5 ppg Milpark's CarboDrill 11-HW mud(designated as Na-7 mud) was prepared. This is a "high water" contentoil mud with an oil-to-water ratio of 50:50. The formulation of this mudis as follows:

    ______________________________________                                        Water               212 gallons                                               Diesel              212 gallons                                               NaCl                588 pounds                                                Carbo-Tec L         11.2 gallons                                              Lime                30 pounds                                                 Carbo-Mul           19.5 gallons                                              Barite              3,800 pounds                                              SurfCote            24 pounds                                                 ______________________________________                                    

The final mud properties and retort analysis on this mud were asfollows:

    ______________________________________                                        Plastic Viscosity, cp                                                                              56                                                       Yield Point, lb/100 ft.sup.2                                                                       37                                                       10 sec/10 nun Gels, lb/100 ft.sup.2                                                                13/15                                                    Electrical Stability, volt                                                                         216                                                      Oil Volume Fraction  0.39                                                     Water Volume Fraction                                                                              0.39                                                     Solids Volume Fraction                                                                             0.22                                                     ______________________________________                                    

Both the oil mud cement use and the universal fluid concept were testedin a large scale test unit which allowed mixing, pumping, filtration,and setting by using field equipment in a realistically sized simulatedborehole at elevated temperatures. The filtration section of the testunit had a five-inch casing centralized (100% standoff) in a 61/2-inchborehole in a synthetic permeable core 15 feet long. Two tests, OMC-1and OMC-2, were run using this test system. A 12-barrel (504 gallons)batch of the above-listed mud was prepared and stabilized by stirringand pumping with high shear within the mixing unit. Portions of this mudwere used for the two tests. The test slurries were batch-mixed, usingthis stabilized mud, with a high shear mixer (RCM cement mixing unit)just prior to their displacement into and circulation through the testmodel using a triplex pump.

In both tests an oil mud-cement slurry was made using a totalsolids-to-oil volume ratio of 1.8 and a total water-to-cement weightratio of 0.31 (563 pounds of Class C cement and 49.3 pounds of addedwater per barrel of mud).

In Test OMC-1 this oil mud-cement slurry was circulated across theclean, permeable section of the model for five minutes and was then leftstatic for five hours to filter at 100 psi pressure differential and150° F. The filtration was then stopped and the temperature raised to200° F. for curing.

In Test OMC-2 a universal fluid was prepared from the same base mud butwith no additional water and only half of the previously used 563 poundsof cement per barrel of mud. This universal fluid was circulated acrossa clean permeable section of another test model and was left to filterfor 18 hours at 150 psi and 150° F. Another portion of the base mud wasthen converted into the final oil mud-cement slurry by adding the sameamounts of cement, water, and accelerators contained in the OMC-1 test.In field usage the universal fluid would have been the drilling mud andthe addition of cement would have been only the differences of thecomponents of the universal fluid and the final OMC slurry. This OMCslurry was used to displace the universal fluid from the model and tocirculate across the universal fluid filter cake for several minutes. Itwas then left to filter at 150 psi and 150° F., forming an oilmud-cement cake on top of the universal filter cake. After thefiltration it was left to cure at 250° F.

This part of the testing demonstrated that both the universal fluid andthe oil mud cement slurries could be mixed, pumped, and set usingfield-type equipment without major operational problems.

Displacement Test Results

The traditional percent "displacement efficiency" is not applicable inthese tests because no normal drilling mud filter cake was formed.Building and incorporating a settable universal fluid filter cake wasthe major goal for the OMC-2 test.

Slurry samples collected during the displacement tests were poured in2-inch cube molds and small shear bond molds (1.5-inch diameter by 7inches long) and cured in a water bath at 200° F. As shown in Table G,both OMC-1 and OMC-2 were demonstrated to gain adequate compressivestrengths and excellent shear bonds.

                  TABLE G                                                         ______________________________________                                        Compressive Strengths and Shear Bonds                                         on OMC-1 and OMC-2 Samples Aged at 200° F.                                     Average Compressive                                                           Strength (psi) Average Shear Bond (psi)                               Test    (crushed)      (shear bond mold)                                      ______________________________________                                        OMC-1   895 (8 days)   Not measured                                           OMC-2   960 (7 days)   258 (7 days)                                           ______________________________________                                    

A slurry sample collected during the OMC-2 test was run on an ultrasoniccement analyzer (UCA). Although the ultrasonic cement analyzer indicateda strength of 534 psi, its core recovered after the ultrasonic cementanalyzer test was crushed, giving a compressive strength of 1108 psi.

Compressive strength data were obtained as checks of the effects of timeand temperature on the oil mud-cement slurry and the heat aging samples.A one lab-barrel (350 ml) sample of the large test batch of the base mudwas made into an oil mud-cement using the same composition as used inthe displacement tests. Small vial samples of this oil mud-cement werethen heat aged at 180° F., 200° F., and 210° F., leaving the mainportion at room temperature. These three samples showed increasedcompressive strength with increased aging temperature (1104, 1304, and1385 psi, respectively, all at 2.8 days curing time).

After three days the main sample was stirred with a Hamilton Beach mixerand another vial sample taken for heat aging. After 10 days anotherstirred sample was taken. After 11 days a non-stirred sample and astirred sample (OMC 701) was taken. All of these samples set hard at180° F. giving an average Brinell compressive strength of 1218 psi.

After 17 days, however, two stirred samples were taken, neither of whichset even after 29 days at 180° F.

In some cases this time related non-setting phenomenon has been reversedby a strong mechanical shearing. In some other cases, the setting by theaddition of more accelerator combined with the strong mechanicalshearing has been required to initiate the test.

Zonal Isolation Tests

Each of the two test models with the solidified oil mud-cementssupporting the 5-inch casing in the permeable 61/2-inch simulatedborehole was cut horizontally (perpendicular to its 15-foot axis) 80inches from the top. The lower sections (from the 80- to the 180-inchlevels) were used for the hydraulic bond tests. The entire length of themodel was used for the shear bond tests.

For the hydraulic bond tests, five sets of two holes each (on oppositesides of the model) were drilled at the 90-, 110-, 130-, 150-, and180-inch levels of the bottom 100-inch-long section, each set of holesbeing perpendicular to the adjacent ones. One hole at each level,spiraling down around the model at 90° increments, was drilled throughthe 103/4-inch outside casing and almost through the permeable,synthetic formation lying just beneath. The second hole at each levelwas drilled through the outside casing, the permeable formation, thefilter cake zone, and almost through the oil mud-cement layer(approaching but not reaching the internal casing).

Epoxy cement was then used to bond small pipe nipples, for use aspressure taps, all the way into the holes, sealing all the layersexposed by the drilling. After tile epoxy had hardened, a hole wasdrilled down the axis of each of the first set of nipples, through theremaining part of the permeable layer, and just into the filter cakelayer, thus exposing the formation-filter cake junction. A hole was alsodrilled down the axis of each of the second set of nipples, through theremaining part of the oil mud-cement layer, and just into the innercasing surface, thus exposing the oil mud-cement casing junction.

The "hydraulic bond" at each of the formation-cake junctions and the oilmud-cement casing junctions was successively measured by injecting waterthrough the pressure tap at a rate of four ml per minute until apressure breakdown was noted. The maximum pressure at each tap wasrecorded as the hydraulic bond.

Following the hydraulic bond tests, the entire model was cut into10-inch lengths (except for the 60-80-inch level section which was left20 inches long). The shear bond tests were then run on each section on ahydraulic press by pressing first on the 5-inch casing to measure theoil mud-cement to pipe bond and then on the set oil mud-cement area tomeasure the oil mud-cement to formation bond.

As shown in Table H, both oil mud-cement systems were demonstrated toprovide good bonding in the displacement models. The data on hydraulicand shear bonds indicate that the oil mud-cements can provide excellentzonal isolation in a borehole.

                  TABLE H                                                         ______________________________________                                        Average Hydraulic and Shear Bond Test Results                                 on OMC-1 and OMC-2 Cores                                                      Hydraulic Bond (psi)                                                                             Shear Bond (psi)                                           Test No.                                                                              Formation Casing   Formation                                                                             Casing                                     ______________________________________                                        OMC-1   180       569      45      66                                         OMC-2   187       429      25      39                                         Average 184       500      35      53                                         ______________________________________                                    

Oil Base Universal Fluid

In the full-scale testing, the LC OMC (containing one-half the amount oftile cement needed for OMC-2 and 8.4 lb/bbl Pozzutec 20) was tested asan oil-base universal fluid. This fluid was circulated in the model andleft pressurized static overnight to deposit a settable filter cake inthe test model. This LC OMC fluid was displaced by circulating the fullOMC-2 slurry in the model at the end of the static filtration period.The entire system was then left static and pressurized to cure atelevate temperatures.

The filter cake laid down by the LC OMC hardened well. As discussedpreviously, the set filter cake plays a critical role in providing zonalisolation was shown in Table H.

A 5-gallon sample of the oil-base universal fluid was taken from thetest batch and used for monitoring long-term rheological stability. Asshown in Table I, rheological properties, slurry density, and electricalstability of this slurry were very stable for nearly 50 days. It appearsthat this fluid would be sufficient for drilling operations, assumingnormal mud engineering care.

                                      TABLE I                                     __________________________________________________________________________    Monitoring of Rheological                                                     Properties of the Oil-Base Universal Fluid                                                     Slurry Properties                                            Fann RPM Dial Reading                                                                          PV  YP Gels                                                                             MW   ES                                            600 300                                                                              200                                                                              100                                                                              6 3 (cp)                                                                              (lb/100 ft.sup.2)                                                                   (lb/gal)                                                                           (volt)                                        __________________________________________________________________________    180 60 43 26 4 3 48  12 4  17.4 252                                           114 65 47 27 4 3 49  16 4  17.4 249                                           104 59 43 25 4 3 45  14 4  17.4 235                                           116 60 43 25 4 3 56   4 3  17.4 --                                            102 59 43 25 4 3 53  16 3  17.4 254                                           105 60 44 25 4 3 54  15 3  17.4 278                                           106 62 45 26 5 3 44  18 3  17.4 281                                           107 62 45 26 5 3 45  17 3  17.4 283                                           108 62 44 26 5 4 46  16 4  17.4 308                                           117 68 50 29 5 4 49  19 4  17.4 287                                           110 64 48 28 5 4 56  18 4  17.4 286                                           120 69 52 30 5 4 51  17 4  17.4 297                                           118 67 50 29 5 4 51  16 4  17.4 265                                           123 71 58 32 5 4 52  16 4  17.4 275                                           120 69 52 30 5 4 52  16 4  17.4 280                                           121 70 52 30 5 4 51  19 4  17.4 268                                           __________________________________________________________________________     Notes:                                                                        MW = Slurry density                                                           ES = Electrical stability                                                     Rheology measured with 10x spring at 150° F.                      

What is claimed is:
 1. A composition for use in drilling and cementing awell comprising the product of:an oil mud admixed with sufficient blastfurnace slag and water to form a slurry having a total solids-to-oilweight ratio of about 1.4 to 2.2 and a blast furnace slag weight ratioof about 0.15 to 0.60, to produce an oil-base universal fluid suitablefor drilling a borehole and laying down a settable filter cake on thewalls of said borehole; an effective amount of accelerator and/orretarder admixable with or contacting the filter cake, the acceleratorand/or retarder being functional to cause the filter cake to harden anda salt selected from the group consisting of sodium chloride, potassiumchloride, zinc chloride, sodium nitrate and ammonium sulfate.
 2. Thecomposition of claim 1 wherein the accelerator is selected from thegroup consisting of nitrate accelerators, sodium silicate, sodiumfluoride, sodium silicofluoride, magnesium silicofluoride, zincsilicofluoride, organic acids, and alcohols.
 3. The composition of claim1 including an effective amount of dodecyl benzene amine sulfonate tofunction as a retarder.
 4. The composition of claim 1 wherein the mudused to drill the well partially forms said slurry, thereby minimizingwaste and reducing mud disposal requirements.
 5. The composition ofclaim 1 wherein the water-to-blast furnace slag is about 0.30 to 0.35.6. The composition of claim 1 wherein the slurry includes an effectiveamount of an emulsifier selected from the group consisting of a modifiedtall oil fatty acid and a tall oil polyamide.
 7. The composition ofclaim 1 wherein the accelerator is selected from the group consisting ofa mixture of formic acid, acetic acid, and methanol; magnesiumsilicofluoride; zinc silicofluoride; and dodecyl benzeneamine sulfonate.